State of the art of multistage trim for control valves has many possible configurations. One of the most common types is composed by concentric drilled hollow cylinders.
This type of trim has several solutions. In a solution, shown in FIGS. 1A and 1B, the stroke is divided into separate axial levels. A central plug slides inside the most inner cylinder from one axial end, corresponding to the valve completely closed position, to the opposite axial end, corresponding to the valve completely open position, of the trim. Along its stroke, the plug progressively opens in series the axial levels of the valve, from the first to the last.
Another solution, is described in EP1408265. In this case, the central plug slides inside the most inner cylinder progressively opening a plurality of holes provided along the inner cylinder, an annular plenum being provided between the inner cylinder and a second outer cylinder provided with a second plurality of holes. The annular plenum communicates with all the holes of the inner and outer cylinders. Passing through the holes of the inner cylinder, a gas expands in the annular plenum and then exit the trim by passing through the holes in the outer cylinder.
Disadvantageous features of these solutions are, respectively to keep the velocity of the fluid under certain limits as the gas expands through the trim, the size of the holes must be increased going from the inlet to the outlet cylinders. This increases the noise at the last stage, corresponding to the most outer cylinder, of the trim. As the plug slides the distribution of the pressure drop among the stages is changed. In particular, at low valve openings, the first stage is subject to very high pressure loss and noise generation. In addition, the jets upstream of the last stage are not fully deviated and masked.
Similar inconveniences are present also in the solutions described in the documents CA1003727 and U.S. Pat. No. 4,617,963. It would be therefore desirable to provide an improved valve for which could maintain the advantages and avoid the inconveniences of both solutions above.